The present invention relates to motor vehicle drivelines and in particular to transmitting power continually to a first wheel set and selectively to a second wheel set.
All-wheel drive (AWD) or four-wheel drive (4WD) systems tend to reduce vehicle fuel economy due to increased driveline parasitic losses, even when the AWD or 4WD system is not activated. Driveline disconnect systems improve fuel economy by disconnecting as many of the driveline rotating parts as possible, as close to the transmission output and the secondary drive wheels as possible, when all-wheel drive is not activated.
In virtually all front-wheel drive (FWD) vehicles and many rear-wheel drive (RWD) vehicles that produce AWD or 4WD, operation in two-wheel drive (2WD) is not automatically provided. In such vehicles, 2WD operation is produced in response to being manually selected by a vehicle operator. But requiring that 2WD operation be manually selected creates an inconvenience for operators, who may expect fully automatic operation of the driveline. Fuel economy is also decreased for operators who leave the vehicle in AWD or 4WD mode, or in vehicles that provide no selectable 2WD operation. A need exists in the industry for a control method that automatically switches between the 2WD and AWD or 4WD modes to save fuel while minimizing or eliminating any disruptions that vehicle occupants might notice.
Sensors exist that may directly read a road in front of vehicle tires for grip but the sensors have a high error rate and are prohibitively expensive to use with motor vehicles. A need exists in the industry to estimate whether conditions need the AWD or 4WD mode engaged.
There may be a time delay between activation of the AWD or 4WD mode and connection of the AWD or 4WD system. A need exists in the industry to detect a need to engage the AWD or 4WD mode in advance of conditions needing the AWD or 4WD mode engaged.